The behavior of water confined in nanoscale spaces, particularly within porous coordination polymers (PCPs), is a crucial area of research with implications for gas storage, separation, and understanding confinement effects. PCPs, constructed from organic ligands and metal ions, offer precise control over pore structure and surface properties, making them ideal for studying confined water. Previous studies on water confinement have identified two regimes: one where a water core exhibits bulk-like properties and a surface layer shows exotic behavior, and another where all water molecules interact with the pore walls, leading to significant deviations from bulk properties. Hydrophobic spaces, like carbon nanotubes (CNTs), have been extensively studied, revealing the possibility of solid-liquid supercritical water at high pressure. This study aims to investigate the behavior of water in hydrophilic 1 nm nanopores of PCPs, where the attractive potential of the walls could significantly increase water density and induce exotic states not observed in the bulk phase. The researchers hypothesized that confinement within the hydrophilic PCPs would lead to a unique water state with a blend of ordered and disordered characteristics, different from either bulk ice or liquid water.

Prior research extensively explored water confinement in spaces smaller than 100 nm. Studies have shown distinct behavior depending on pore size. In larger pores (2 nm to tens of nanometers), a core of bulk-like water exists alongside a surface layer with altered properties. Smaller pores (less than 2 nm) see all water molecules strongly influenced by the walls. Carbon nanotubes (CNTs) are frequently used as model systems, primarily focusing on hydrophobic confinement. Simulations predicted the existence of supercritical water in hydrophobic nano-environments under high pressure and ambient temperatures, exhibiting a unique blend of solid and liquid properties due to restrictions on structural symmetry, which could allow the existence of a solid-liquid critical point not found in bulk water. This prior work established a foundation for exploring the potential for exotic water states in various nano-confined environments, particularly those with different surface chemistries.

The researchers synthesized a water-adsorbing PCP composed of oxime ligands and isophthalates. Water adsorption/desorption isotherms were measured at 298 K. Single-crystal X-ray diffraction (SXRD) was employed to determine the structure of water within the PCP, specifically focusing on the arrangement of oxygen atoms. Infrared (IR) spectroscopy was used to analyze the OH stretching modes of the confined water molecules, providing information on hydrogen bond strengths. The IR spectra were obtained using a Fourier Transform Infrared (FTIR) microscope. Isotopic substitution (H₂O with D₂O) was used to assist in peak assignments within the IR spectra. Time-lapse IR spectroscopy was used to monitor changes in the spectra after exposing the sample to D₂O vapor, allowing the determination of the diffusivity of water within the nanostructures. Scanning electron microscopy (SEM) was used to determine channel lengths. Detailed experimental procedures, including sample preparation, data acquisition and analysis methods, including peak fitting techniques and the calculation of diffusivity from time-lapse IR data are provided in the supplementary materials. Data handling and quality control measures were strictly followed to enhance the reproducibility and validity of the study.

The synthesized PCP, PCP-1, exhibited sharp adsorption/desorption of two water molecules per copper ion (guest water) at a relative pressure of 0.04. SXRD analysis revealed an ordered arrangement of oxygen atoms from both guest and coordinated water molecules within the 1 nm channels of PCP-1, forming a square lattice structure. However, IR spectroscopy showed a significant fraction (17%) of broken hydrogen bonds, indicating a liquid-like characteristic. The overall density of water in PCP-1 was comparable to liquid water (approximately 1 g/cm³). The mean number of hydrogen bonds per hydrogen atom (0.83) was intermediate between that of ice and liquid water, indicating the presence of water in an exotic state not found in bulk water's phase diagram. Time-lapse IR spectroscopy, after exposure to D₂O vapor, revealed a diffusivity of water within the PCP channels at least 0.6 × 10⁻¹⁹ m²/s, significantly faster than that observed in ice at the same temperature. The combined SXRD and IR data revealed that water molecules in PCP-1 show a structure similar to that of the solid-liquid supercritical water predicted in previous simulations of water confined within CNTs. Despite the ordered arrangement of water oxygen atoms, a significant fraction of broken hydrogen bonds was observed, suggesting a balance between solid-like ordering and liquid-like disorder.

The observed duality of the confined water in PCP-1 – ordered structure with broken hydrogen bonds and high diffusivity – points to an exotic state not represented in the bulk water phase diagram. This unusual state shares structural and dynamical similarities with solid-liquid supercritical water simulated in 1 nm diameter CNTs. The intermediate number of broken hydrogen bonds aligns with the behavior expected above the predicted solid-liquid critical point in confined systems. The observed square-lattice arrangement of oxygen atoms mirrors predictions for the supercritical regime. The hydrophilic nature of the PCP walls is significant, as it introduces strong attractive potentials that promote the ordered structure while not being strong enough to eliminate broken hydrogen bonds, leading to the observed combination of ice-like order and liquid-like disorder. However, the exact nature of the exotic state – specifically whether it represents solid-liquid supercritical water at ambient pressure - requires further investigation. This discovery opens new avenues for studying the predicted solid-liquid critical point in water under ambient conditions, bypassing the need for extreme pressures.

This research demonstrated the existence of an exotic state of water within hydrophilic nanopores of a PCP, exhibiting a unique combination of ordered structure and broken hydrogen bonds. The properties of this confined water closely resemble those predicted for solid-liquid supercritical water in CNT simulations, offering a new experimental pathway to study this elusive critical point at ambient pressure. Future work should focus on clarifying the exact nature of this state, further exploring the influence of PCP characteristics (pore size, surface chemistry) on the properties of confined water, and exploring potential applications of this unique water phase in controlling chemical reactions.

The study primarily focuses on a single type of PCP. Further investigations using PCPs with different pore sizes, shapes, and surface functionalities are needed to assess the generality of the observed phenomena. While the diffusivity of water was estimated, determining the precise value is hindered by the unknown surface permeability of the PCP. The exact nature of the observed state, whether it indeed corresponds to solid-liquid supercritical water, requires further experimental verification using various techniques to measure pressure and temperature dependence.